The later type bushes or bodies dont have a threaded portion for a nut to hold it in the gearbox cover. The original late type were mazac but the repros are brass. The earlier ones which take securing nut were brass.
You need to remove the cover to remove the speedo drive and replace it.
With cover off and securing pin removed attach a length of old cable to the drive, clamp cable in vice and pull or tap off the cover.
The O ring on the shaft, if fitted , is quite a flimsy item, easily damaged when replacing.
Photo is a new old stock assembly. A sealing ring goes in the goove nearest the thread. Most driven gears dont have provision for an o ring and rely on fit and scroll to keep the oil inside.